1. Field of the Invention
The present invention relates to a quantum cascade laser using intersubband transition in a quantum well structure.
2. Related Background Art
Light with a mid-infrared wavelength region (for example, wavelength of 5 to 30 μm) is an important wavelength region in the field of spectroscopy analysis. As a high-performance semiconductor light source in this wavelength region, in recent years, quantum cascade lasers (QCL) have gained attention (for example, refer to Patent Documents 1 to 3 and Non-Patent Documents 1 and 2).
A quantum cascade laser is a monopolar type laser element which uses a level structure including subbands formed in a semiconductor quantum well structure and generates light by means of intersubband electron transition, and can realize high efficiency and high-output operations by multistage cascade-coupling of quantum well emission layers which are formed by quantum well structures and become active regions. Further, this cascade coupling of quantum well emission layers is realized by using electron injection layers for injecting electrons into emission upper levels and alternately laminating quantum well emission layers and injection layers.    Patent Document 1: US Patent Publication No. 5457709    Patent Document 2: Japanese Patent Application Laid-Open No. 2009    Patent Document 3: Japanese Patent Application Laid-Open No. 2008-10733    Patent Document 4: Japanese Patent Application Laid-Open No. H5-110188    Patent Document 5: Japanese Patent Application Laid-Open No. H10-200195    Non-Patent Document 1: S. Blaser et al., “Room-temperature, continuous-wave, single-mode quantum-cascade lasers at λ=5.4 μm,” Appl. Phys. Lett. Vol. 86 (2005), pp. 041109-1 to 041109-3    Non-Patent Document 2: J. S. Yu et al., “High-power, room-temperature, and continuous-wave operation of distributed-feedback quantum-cascade lasers at λ-4.8 μm,” Appl. Phys. Lett. Vol. 87 (2005), pp. 041104-1 to 041104-3